This invention is concerned with a method for producing an optically readable data memory, such as a compact disc (CD), having a textured surface. The data memory can be used for storage of information in electronic data processing and in entertainment electronics, for example.
In a known mechanical production method for CD's, a die is pressed onto a substrate material which, typically, is still fluid in an injection molding operation. Pressing causes a desired change in surface texture or morphology. Upon suitable further treatment, a specific change is realized in optical reflection properties which can be scanned, without contact, by a laser beam of small diameter. With mechanical production of surface texture on the substrate, wear on the die is unavoidable, however, and necessitates repeated replacement of the die for large production runs. Another disadvantage of the known mechanical production method lies in that a texture pattern cannot be changed on short notice.
It is also known that information can be stored on an optically readable data medium by selective removal of a memory layer by means of an intense, focused laser beam. This method, however, requires costly high-power lasers and optics and/or special, easily vaporized coating materials. Because of the limited depth of field of the optical image at the high resolution required, this method is limited either to sequential texturing with continual refocusing, or to extremely flat substrate surfaces whose planeness far exceeds what is currently attainable for industrial plastic surfaces.
It is known further that ultraviolet (UV) absorptivity of polymethylmethacrylate (PMMA) can be locally enhanced by selective irradiation with UV light, and that the regions of enhanced absorptivity then can be selectively removed by ablation with locally non-selective exposure to longer-wavelength radiation; see S. Kuper and M. Stuke, "UV-Excimer-Laser Ablation of Polymethylmethacrylate at 248 nm: Characterization of Incubation Sites with Fourier Transform IR- and UV-Spectroscopy," Applied Physics A, Vol. 49, pp. 211-215 (1989). The cited paper is concerned with scientific investigation of the mechanism of ablation, is unconcerned with practical applications, and does not show the high degree of spatial resolution required for practicality.
It is an object of the present invention to produce a CD without the use of a mechanical die.
Another object of the invention with regard to an optically readable data memory, such as a CD, is to enable changing a texture pattern on short notice.